This invention relates to an improved process for making esters. More particularly, the invention relates to an economical and efficient process for accelerating the rate of esterification in which dicarboxylic acids or their esters and monohydroxy aromatic compounds are reacted to form esters.
Linear polyesters which are essentially the product of reaction of a bisphenol and dicarboxylic acids are important thermoplastic materials due to their excellent performance at high temperatures. It is known to produce such linear polyesters by first reacting at least one dicarboxylic acid with a monohydroxy aromatic compound to form a diaryl ester of the dicarboxylic acid, and then carry out a transesterification polymerization between the diaryl ester and a bisphenol. See, e.g., British Pat. No. 924,607 and U.S. Pat. Nos. 4,255,555 and 4,319,017. However, those references are primarily concerned with transesterification reaction for preparing the linear polyesters.
Significant commercial interest in recent years has developed in the field of phenolic esters of aromatic dicarboxylic acids, such as diphenyl isophthalate and diphenyl terephthalate, due to their use in many processes. For example, mixtures of diphenyl isophthalate and diphenyl terephthalate can be reacted by melt polycondensation with 4,4'-(1-methyl-ethylidene) bis(phenol) to produce aromatic polyesters or polyarylates. Diphenyl phthalates can also be reacted with primary amines in a solvent to make polyamides. Likewise, 3,3' diaminobenzidene may be condensed with various diphenyl esters to form polybenzimidazoles.
The prior art processes for preparing the diaryl esters suffer from a nubmer of disadvantages. To obtain a degree of esterification in excess of 90%, which is demanded by the economics of the processes, the prior art processes require lengthy reaction times. Diaryl esters substantially free of dicarboxylic acid may also be required for the production of high quality linear polyesters. Other disadvantages include a requirement for high reaction temperatures, i.e. in the range of 280.degree.-300.degree. C. Such relatively high temperatures not only consume more energy than reactions conducted at lower temperatures, but they also result in a darker colored product which may be contaminated with by-products from side reactions. Other prior art processes also utilize pressures in excess of 100 psig and approaching 200 psig, which is again more costly and increases safety hazards.
U.S. Pat. No. 4,124,566 discloses a process for preparing polyesters in which the first step is the esterification of a difunctional carboxylic acid with an aromatic monohydroxy compound and an aliphatic diol and/or a dihydroxybenzene. The esterification reaction is to be performed in the presence of an aromatic hydrocarbon medium. This disclosure of the use of a small portion of aromatic esters as solvent for the reaction as well as the use of an azeotrope, including aromatic hydrocarbons such as ethyl benzene, affords some relief from lengthy reaction cycles and low conversion rates. However, the addition of the aromatic esters in the esterification reaction mixture reduces the volume available for the reaction and, consequently, reduces yield per batch. The addition of an azeotropic agent also dilutes the reaction mixture, and it may adversely affect the solubility of the dicarboxylic acid in the mixture. The use of an azeotropic agent such as ethyl benzene or xylene also increases the potential for fire and explosion, and the added danger of environmental health hazards to workers exposed to the atmosphere. Thus, the use of such aromatic hydrocarbons requires careful monitoring and treatment to prevent contamination of the environment. Accordingly, there is a need for an improved process for making diaryl esters of dicarboxylic acids.
It is, therefore, an object of the invention to provide improved process for making diaryl esters of dicarboxylic acids.
It is another object of the present invention to provide an economical and environmentally safe process for making diaryl esters which permits relatively short reaction times and high conversion rates while operating at relatively low reaction temperatures and pressures.
These and other objects of the invention can be gathered from the following disclosure.